Understanding Instrumental Errors in Levelling

Levelling is a surveying technique used to determine the relative heights of different points on the ground. During the levelling process, various types of errors can occur which affect the accuracy of the results. These errors can be broadly classified into instrumental errors, personal errors, and natural errors.

What are Instrumental Errors?

Instrumental errors are those errors that arise due to imperfections or faulty adjustments in the surveying instruments themselves. These are inherent problems with the equipment being used for levelling.

Analyzing Levelling Errors from Options

Let's examine the given options to identify which one represents an instrumental error during levelling:

• Error in sighting: This can sometimes be personal (e.g., difficulty focusing) or instrumental (e.g., parallax if the instrument is not properly focused or adjusted). However, the phrasing "Error in sighting" is broad and can include personal elements.
• Rod not of standard length: The levelling staff (or rod) is a key piece of equipment used along with the level. If the staff is not manufactured to the correct standard length, or if it has been damaged or repaired incorrectly, all readings taken on it will be systematically wrong. This error originates from the measuring instrument itself (the rod), making it an instrumental error.
• Mistake in rod handling: This refers to errors made by the person holding the levelling staff. Examples include holding the staff tilted instead of vertically, or not holding it firmly on the benchmark or ground point. These are personal errors, not instrumental ones.
• Mistakes in manipulation: This relates to errors made by the surveyor or the staff holder during the process of setting up the instrument, taking readings, or recording data. Examples include not properly levelling the instrument, misreading the staff, or incorrectly booking the values. These are personal errors.

Based on the analysis, an error where the levelling rod not of standard length directly relates to a defect or inaccuracy in the instrument (the rod) itself. Therefore, this is classified as an instrumental error in levelling.